Air/fuel ratio feedback control system for an internal combustion engine

Abstract: An air/fuel control system which includes feedback control from an exhaust gas oxygen sensor (44) positioned upstream of a catalytic converter (50). The exact air/fuel ratio required optimum converter efficiency is determined by generating an emissions signal from both a HC/CO sensor (54) and a NO sensor (46) each positioned downstream of the catalytic converter (50). The feedback variable (FV) is trimmed by a signal derived from the emissions signal to maintain air/fuel operation at a value corresponding to maximum converter efficiency.

Description: The invention relates to air/fuel control systems for internal combustion engines equipped with catalytic converters. Feedback control systems are known for trimming liquid fuel delivered to an internal combustion engine in response to an exhaust gas oxygen sensor positioned upstream of a three-way catalytic converter. Typically, the exhaust gas oxygen sensor provides a two-state, high/low (rich/lean) output dependent upon the existence of a low or high oxygen partial pressure in the engine exhaust under local thermodynamic equilibrium on the sensor electrodes. Because the exhaust gas may not be in thermodynamic equilibrium, the high-to-low switch point of the sensor may not occur at the stoichiometric air/fuel ratio. In particular, the switch point may not coincide exactly with the peak of the window of the three-way catalytic converter. It is also known to use a second EGO sensor downstream of the catalytic converter for the purpose of reducing the mismatch between the sensor switch point and the peak window of the catalytic converter by biasing the mean air/fuel value. The inventors herein have recognized, however, that even though an exhaust gas oxygen sensor positioned downstream of a catalytic converter provides a better indication of the catalytic converter operating window than an upstream sensor, it may not always provide the desired indication. Even when a relatively good correspondence is initially achieved, aging and temperature affects of the downstream oxygen sensor may cause a variance between the sensor indication and the air/fuel ratio required for maximum efficiency of the catalytic converter. The inventors herein have also found that even when the post catalytic oxygen sensor accurately switches at stoichiometry, the switch point may not be accurately aligned with the most efficient converter efficiency for a particular converter. EP-A-310 120 describes an electronic air/fuel ratio control apparatus in an engine having a ternary catalyst in the exhaust system. The control system includes, upstream of the ternary catalyst, an exhaust gas oxygen sensor incorporating a nitrogen oxide reducing catalyst layer. The air/fuel ratio is controlled to a first value in dependence upon the detection of oxygen concentration including the oxygen in the nitrogen oxides. The air/fuel ratio is re-set to a richer value if a high concentration of nitrogen oxides is detected or to a leaner value when a high concentration of incompletely burnt components is detected. JP-A-02 125 941 describes an air/fuel control device in which an exhaust gas oxygen sensor is positioned upstream of a ternary catalyst and a nitrogen oxide sensor and a carbon monoxide sensor are positioned downstream of the catalyst. The air/fuel ratio is controlled to be weaker when the carbon monoxide density exceeds its set level and to be richer when the nitrogen oxide density exceeds its set level. An object of the invention herein is to provide engine air/fuel operation within the operating window of the any catalytic converter coupled to the engine exhaust regardless of the air/fuel location of the converter's operating window. The above object is achieved, and disadvantages of prior approaches overcome, by providing both a control system and method for optimizing conversion efficiency of a catalytic converter positioned in the engine exhaust according to the independent claims. In one particular aspect of the invention, the control method comprises the steps of: measuring nitrogen oxide content of exhaust gases downstream of the catalytic converter to generate a first measurement signal, measuring combined hydrocarbon and carbon monoxide content in exhaust gases downstream of the catalytic converter to generate a second measurement signal, subtracting the first measurement signal from the second measurement signal to generate a third signal, generating a correction signal from an exhaust gas oxygen sensor positioned upstream of the catalytic converter, trimming the correction signal with a trim signal derived from the third signal and then integrating to generate a feedback variable, and correcting fuel delivered to the engine by the feedback variable to maintain maximum conversion efficiency of the catalytic converter. An advantage of the above aspect of the invention is that engine air/fuel operation is achieved at an air/fuel ratio which results in maximum catalytic converter efficiency regardless of the converter used. This advantage is obtained while maintaining rapid air/fuel corrections. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an embodiment wherein the invention is used to advantage; Figure 2 is a high level flowchart of various operations performed by a portion of the embodiment shown in Figure 1; Figures 3A-3D represent various electrical waveforms generated by a portion of the embodiment shown in Figure 1 and further described in Figure 2; Figure 4 is a high level flowchart of various operations performed by a portion of the embodiment shown in Figure 1; and Figure 5 is graphical representation of normalized emissions passing through a catalytic converter as a function of engine air/fuel operation. Controller 10 is shown in the block diagram of Figure 1 as a conventional microcomputer including: microprocessor unit 12; input ports 14; output ports 16; read-only memory 18, for storing the control program; random access memory 20 for temporary data storage which may also be used for counters or timers; keep-alive memory 22, for storing learned values; and a conventional data bus. Controller 10 is shown receiving various signals from sensors coupled to engine 28 including; measurement of inducted mass airflow (MAF) from mass airflow sensor 32; manifold pressure (MAP), commonly used as an indication of engine load, from pressure sensor 36; engine coolant temperature (T) from temperature sensor 40; indication of engine speed (rpm) from tachometer 42; indication of nitrogen oxides (NOx) in the engine exhaust from nitrogen oxide sensor 46 positioned downstream of three-way catalytic converter 50; and a combined indication of both HC and CO from sensor 54 positioned in the engine exhaust downstream of catalytic converter 50. In this particular example, sensor 54 is a catalytic-type sensor sold by Sonoxco Inc. of Mountain View, California and sensor 46 is a nitrogen dioxide Saw-Chemosensor as described in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, VOL. UFFC-34, NO. 2, March 19, 1987, pgs. 148-155. The invention may also be used to advantage with separate measurements of HC and CO by separate hydrocarbon and carbon monoxide sensors. In addition, controller 10 receives two-state (rich/lean) signal EGOS from comparator 38 resulting from a comparison of exhaust gas oxygen sensor 44, positioned upstream of catalytic converter 50, to a reference value. In this particular example, signal EGOS is a positive predetermined voltage such as one volt when the output of exhaust gas oxygen sensor 44 is greater than the reference value and a predetermined negative voltage when the output of sensor 44 switches to a value less than the reference value. Under ideal conditions, with an ideal sensor and exhaust gases fully equilibrated, signal EGOS will switch states at a value corresponding to stoichiometric combustion. Intake manifold 58 of engine 28 is shown coupled to throttle body 59 having primary throttle plate 62 positioned therein. Throttle body 59 is also shown having fuel injector 76 coupled thereto for delivering liquid fuel in proportion to the pulse width of signal fpw from controller 10. Fuel is delivered to fuel injector 76 by a conventional fuel system including fuel tank 80, fuel pump 82, and fuel rail 84. Referring now to Figure 2, a flowchart of a routine performed by controller 10 to generate fuel trim signal FT is now described. A determination is first made whether closed-loop air/fuel control is to be commenced (step 104) by monitoring engine operating conditions such as temperature. When closed-loop control commences, sensor 54 is sampled (step 108) which, in this particular example, provides an output signal related to the quantity of both HC and CO in the engine exhaust. The HC/CO output of sensor 54 is normalized with respect to engine speed and load during step 112. A graphical representation of this normalized output is presented in Figure 3A. As described in greater detail later herein, the zero level of the normalized HC/CO output signal is correlated with the operating window, or point of maximum converter efficiency, of catalytic converter 50. Continuing with Figure 2, nitrogen oxide sensor 46 is sampled during step 114 and normalized with respect to engine speed and load during step 118. A graphical representation of the normalized output of nitrogen oxide sensor 46 is presented in Figure 3B. The zero level of the normalized nitrogen oxide signal is correlated with the operating window of catalytic converter 50 resulting in maximum converter efficiency. During step 122, the normalized output of nitrogen oxide sensor 46 is subtracted from the normalized output of HC/CO sensor 54 to generate combined emissions signal ES. The zero crossing point of emission signal ES (see Figure 3D) corresponds to the actual operating window for maximum converter efficiency of catalytic converter 50. As described below with reference to process steps 126 to 134, emission signal ES is processed in a proportional plus integral controller to generate fuel trim signal FT for trimming feedback variable FV which is generated as described later herein with respect to the flowchart shown in Figure 4. Referring first to step 126, emission signal ES is multiplied by gain constant GI and the resulting product added to the products previously accumulated (GI * ES i-1 ) in step 128. Stated another way, emission signal ES is integrated each sample period (i) in steps determined by gain constant GI. During step 132, emission signal ES is also multiplied by proportional gain GP. The integral value from step 128 is added to the proportional value from step 132 during addition step 134 to generate fuel trim signal FT. In summary, the proportional plus integral control described in steps 126-132 generates fuel trim signal FT from emission signal ES. The routine executed by microcomputer 10 to generate the desired quantity of liquid fuel delivered to engine 28 and trimming this desired fuel quantity by a feedback variable related both to EGO sensor 44 and fuel trim signal FT is now described with reference to Figure 4. During step 158, an open-loop fuel quantity is first determined by dividing measurement of inducted mass airflow (MAF) by desired air/fuel ratio AFd which is typically the stoichiometric value for gasoline combustion. This open-loop fuel charge is then trimmed, in this example divided, by feedback variable FV. After a determination that closed-loop control is desired (step 160) by monitoring engine operating conditions such as temperature, signal EGOS is read during step 162. During step 166, fuel trim signal FT is transferred from the routine previously described with reference to Figure 2 and added to signal EGOS to generate trim signal TS. During steps 170-178, a conventional proportional plus integral feedback routine is executed with trimmed signal TS as the input. Trimmed signal TS is first multiplied by integral gain value KI (see step 170) and this product is added to the previously accumulated products (see step 172). That is, trimmed signal TS is in steps determined by gain constant KI each sample period (i). This integral value is added to the product of proportional gain KP times trimmed signal TS (see step 176) to generate feedback variable FV (see step 178). As previously described with reference to step 158, feedback variable FV trims the fuel delivered to engine 28. Feedback variable FV will correct the fuel delivered to engine 28 in a manner to drive emission signal ES to zero. An example of operation for the above described air/fuel control system is shown graphically in Figure 5. More specifically, measurements of HC, CO, and NOx emissions from catalytic converter 50 after being normalized over an engine speed load range are plotted as a function of air/fuel ratio. Maximum converter efficiency is shown when the air/fuel ratio is increasing in a lean direction, at the point when CO and HC emissions have fallen near zero, but before NOx emissions have begun to rise. Similarly, while the air/fuel ratio is decreasing, maximum converter efficiency is achieved when nitrogen oxide emissions have fallen near zero, but CO and HC emissions have not yet begun to rise. In accordance with the above described operating system, the operating window of catalytic converter 50 will be maintained at the zero crossing point of emissions signal ES (see Figure 3D) regardless of the reference air/fuel ratio selected and regardless of the switch point of EGO sensor 44. An example of operation has been presented wherein emission signal ES is generated by subtracting the output of a nitrogen oxide sensor from a combined HC/CO sensor and thereafter fed into a proportional plus integral controller. The invention claimed herein, however, may be used to advantage with other than a proportional plus integral controller. The invention claimed herein may also be used to advantage with separate HC and CO sensors and either the CO or the HC sensor may be used in conjunction with the nitrogen oxide sensor. And, the invention may be used to advantage by combining the sensor outputs by signal processing means other than simple subtraction.

Claims: An engine air/fuel control method for optimizing conversion efficiency of a catalytic converter (50) positioned in the engine exhaust, comprising the steps of: measuring nitrogen oxide content (114) of exhaust gases downstream of the catalytic converter to generate a first measurement signal; measuring combined hydrocarbon and carbon monoxide content (108) in exhaust gases downstream of the catalytic converter to generate a second measurement signal; subtracting said first measurement signal from said second measurement signal (122) to generate a third signal; generating a correction signal (EGOS) from an exhaust gas oxygen sensor (44) positioned upstream of the catalytic converter (50); trimming said correction signal with a trim signal (FT) derived from said third signal and then integrating to generate a feedback variable (FV); and correcting fuel delivered to the engine (28) by said feedback variable (FV) to maintain maximum conversion efficiency of the catalytic converter. A method according to claim 1, further comprising the step of integrating said third signal to derive said trim signal. A method according to claim 2, further comprising the step of multiplying said third signal by a proportional term and adding the resulting product to said integration of said third signal to derive said trim signal. An engine air/fuel control method for optimizing conversion efficiency of a catalytic converter positioned in the engine exhaust, comprising the steps of: measuring nitrogen oxide content of exhaust gases downstream of the catalytic converter and normalizing said measurement with respect to at least engine speed to generate a first measurement signal; measuring combined hydrocarbon and carbon monoxide content in exhaust gases downstream of the catalytic converter and normalizing said measurement with respect to at least engine speed to generate a second measurement signal; subtracting said first measurement signal from said second measurement signal to generate a trim signal; generating a correction signal from an exhaust gas oxygen sensor positioned upstream of the catalytic converter; trimming said correction signal with said trim signal and then integrating to generate a feedback variable; delivering fuel to the engine in response to an indication of airflow inducted into the engine and a reference air/fuel ratio; and correcting said delivered fuel by said feedback variable to maintain maximum conversion efficiency of the catalytic converter. A method according to claim 4, wherein said trim signal is derived by integrating said emissions indicating signal and adding a product of a gain value times said emissions indicating signal to the resulting integration. A method according to claim 4, wherein said step of generating a correction signal further comprises a step of comparing said exhaust gas oxygen sensor output to a reference value such that said correction signal has a predetermined amplitude with a first polarity when exhaust gases are rich of a preselected air/fuel ratio and a second polarity opposite said first polarity when said exhaust gases are lean of said preselected air/fuel ratio. An engine control system for optimizing conversion efficiency of a catalytic converter positioned in the engine exhaust, comprising: a first sensor positioned downstream of the catalytic converter for providing a first electrical signal having an amplitude related to the quantity of nitrogen oxides in the exhaust; a second sensor positioned downstream of the catalytic converter for providing a second electrical signal having an amplitude related to the combined hydrocarbon and carbon monoxide content in the exhaust; means to subtract said first electrical signal from said second electrical signal to generate a third signal; an exhaust gas oxygen sensor positioned upstream of the catalytic converter for generating a correction signal; trimming means to trim said correction signal with a trim signal derived from said third signal and to generate a feedback variable by integration; and fuel control means for delivering fuel to the engine in relation to the feedback variable. An engine control system as claimed in claim 7, wherein integrating means are provided to integrate said third signal to derive said trim signal. An engine control system as claimed in claim 8, wherein multiplying means are provided to multiply said third signal by a proportional term and to add the resulting product to the integration performed by the integrating means. An engine control system as claimed in claim 7, 8 or 9, further comprising normalizing means for normalizing said first electrical signal and said second electrical signal with respect to engine speed and engine load.

Inventors: HAMBURG DOUGLAS RAY, LOGOTHETIS ELEFTHERIOS MILTIAD, SOLTIS RICHARD E, VISSER JACOBUS HENDRIK, HAMBURG, DOUGLAS RAY, LOGOTHETIS, ELEFTHERIOS MILTIADIS, SOLTIS, RICHARD E., VISSER, JACOBUS HENDRIK
Applicants: FORD FRANCE, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S.A., FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1994
Main classification: F02D41
IPC codes: F02D35, F01N3, F02D41